1. Field of the Invention
This invention relates generally to the field of expansion bolts. More particularly, the present invention relates to the class of expansion bolts, known as "roof bolts", that are anchored in rock formations in underground tunnels and the like to provide structural support for the roof of the tunnel.
2. Brief Description of the Prior Art
Expansion bolts and similar devices have long been used in mining operations to provide strutural support for the ceiling or roof of a mine tunnel and to support loads (such as slusher buckets, for example) that are suspended from the tunnel roof. Of primary importance, however, is the function of supporting the mine roof against collapse. To this end, a long hole of predetermined diameter is drilled into the structural layers of rock and the like forming the roof of the tunnel. These holes are usually several feet in length, with seven feet (or somewhat more than 2 meters) being a typical length. An expansion bolt is inserted into the hole, the expansion bolt having an expansion portion at the innermost, or distal end of the hole, and a rock plate surrounding the outer opening of the hole.
Proper installation of a roof bolt is critical to its proper functioning. To this end, it is necessary to assure that the bolt engages the walls of the pre-drilled hole with substantially uniform force throughout the circumference of the expansion portion, thereby minimizing the chances of slippage Proper design of the expansion portion, as well as proper tightening of the bolt, help accomplish this function.
Prior art expansion bolts typically employ a split sleeve as the expansion portion, with the split portions of the sleeve being cammed outwardly by a wedge member as the bolt is tightened. The outwardly-cammed sleeve portions engage the walls of the hole. A problem with many expansion bolts of this general design is that of assuring an even application of pressure around the circumference of the expansion sleeve. For example, one split portion may deform excessively as the bolt is tightened. The result may be reduced stability and an increased possibility of slippage.
Another problem frequently encountered with prior art roof bolts stems from the fact that such bolts must perform two functions that may be at odds with one another. The first function is that of placing a radial pressure against the walls of the hole at or near its distal end. The second function is that of applying a slight pressure between the deeply anchored distal end of the bolt and its outer or proximal end, thereby placing the rock layer or layers there between in compression, with a corresponding longitudinal tension being placed on the bolt itself. These two functions may conflict, for example, if the expansion portion of the bolt locks it into a longitudinal position wherein there is insufficient longitudinal tension on the bolt. This condition would result if the head (proximal end) of the bolt were not drawn snugly against the rock shoulder surrounding the opening of the pre-drilled hole before the bolt became anchored in the hole by its expansion portion.
A roof bolt designed to overcome these problems is disclosed and claimed in U.S. Pat. No. 4,173,918, issued to Raymond Piersall, the applicant of the present application. The roof bolt disclosed and claimed in this previous patent includes an elongate stud having distal and proximal threaded ends, with a cylindrical, axially slotted sleeve mounted near the distal end. The distal end carries a frusto-conical wedge adapted to enter the distal end of the sleeve as the bolt is tightened, forcing the slotted sleeve portions to expand outwardly in response to the relation of the stud. The proximal end of the stud carries a nut, which is attached to the stud so as to be held in a fixed rotational position relative thereto until a predetermined torque is applied to the nut. A rock plate is carried on the stud between the nut and the sleeve. This design assures a uniform radial pressure applied by the expansion sleeve. In addition, the attachment of the nut to the stud allows the bolt to be longitudinally tensioned, by means of the pressure applied by the rock plate around the opening of the hole, after its expansion sleeve has been properly located and anchored.
Although the above-described device accomplishes its desired purpose, further improvements are desirable. For example, the wedge member in the above-described invention comprises a solid piece of steel alloy, machined to its proper dimensions and its frusto-conical shape. The need to fabricate this element by machining adds greatly to the cost of the roof bolt. Secondly, in some applications, a greater degree of expansion may be desired than can be easily accomplished by the wedge mechanism. With lower cost, and greater versatility, the range of applications for a roof bolt in accordance with this general concept can be expanded.